1. Field of the Disclosure
The disclosure relates to an apparatus for fabrication of gallium nitride (GaN) bulk single crystal and a fabrication method of GaN single crystal ingot using the same; and, more particularly, to the apparatus for fabrication of GaN bulk single crystal which is capable of epitaxially growing GaN bulk single crystal by enhancing availability and deposition efficiency of GaN source gas, and to the fabrication method of GaN single crystal ingot using the same.
2. Description of the Related Art
Gallium nitride (GaN) as a wide bandgap semiconductor with a direct transition bandgap of which the bandgap energy is 3.39 eV, has been researched since beginning of the 1970s in order that it may be applied to a protection film and various optoelectronic devices such as a blue light emitting diode (LED) or the like. At room temperature, GaN crystal includes a plurality of unit cells with predetermined lattice constants of which “a” parameter and “c” parameter is 3.189 Å and 5.185 Å, respectively. Furthermore, since nitrogen has high electronegativity, GaN has a crystal structure of wurzite in a stable state and has a crystal structure of zinc-blende in a metastable state.
However, absence of a substrate material is still a major problem when developing an optical device using GaN. The best way to overcome this problem is to develop a GaN single crystal ingot, which is utilized as the substrate. But, since process conditions for growing the GaN single crystal ingot are very difficult, only GaN single crystal having a size of about 5 mm has been reported to date. In order to use a thick GaN film as the substrate, a research for fabricating a freestanding GaN substrate is being conducted. That is, according to this research, a sacrificial film such as zinc oxide (ZnO) is deposited on a different substrate such as sapphire, and then thick GaN film is grown on the sacrificial film. Thereafter, the sacrificial film is removed so that a desired thick GaN film may be obtained.
A hydrogen vapor phase epitaxial (HVPE) growing method is primarily utilized for growing the thick GaN film, because it has several advantages. That is, according to the HVPE growing method, epitaxial growth proceeds rapidly at a speed ranging from 50 μm/hr to 100 μm/hr and further, high purity epitaxial film can be grown during thin film growth. In addition, since fabrication of an apparatus is simpler than those required in other epitaxial growing methods, thick GaN film can be fabricated at a low manufacturing cost.
FIG. 1 is a flow chart illustrating a fabrication process of a GaN substrate using a prior art HVPE method.
In operation 2, a thick GaN film with a thickness ranging from 300 μm to 500 μm is grown on a sapphire substrate. Afterwards, in operation 4, a laser beam is irradiated on a GaN surface through the sapphire substrate using an excimer laser or an yttrium-aluminum-garnet (YAG) laser, to separate the thick GaN film from the sapphire substrate. Thereafter, in operation 6, the separated thick GaN film is processed to have a circular shape with a predetermined diameter and then, a hardfacing process is performed on a surface of the resultant thick GaN film. Finally, in operation 8, a damage layer remaining upon the surface of the thick GaN film which has been experienced through the hardfacing process is removed using a dry etching process.
U.S. Pat. No. 6,632,725 B2 discloses a crystal-growing method of a GaN epitaxial layer by means of the HVPE method. However, according to the HVPE method employing a chemical vapor deposition (CVD) technique of an open type, the amount of the GaN crystal grown on the sapphire substrate is exceedingly small with respect to total amount of GaN which is produced by reaction of GaCl and NH3 gas so that manufacturing cost is increased.
Typically, the weight percent ratio of GaN crystal grown on the sapphire substrate with respect to the total GaN is not more than 5%. That is, most of GaN is attached on inner walls of a reactor or flows out with the exhaust, which causes a high manufacturing cost. Furthermore, when growing GaN on the sapphire substrate by the above HVPE method, it is shown that the defect density of GaN film within 1 μm thickness is beyond about 1010/cm2 because of lattice constant inconformity between the sapphire substrate and the GaN film. On the contrary, when growing the GaN film up to a predetermined thickness more than 500 μm, defect density is decreased to about 106/cm2. Therefore, the GaN substrate obtained after crystal growth is deformed due to the defect density difference between a front face and a rear face. Generally, since the rear face shows a high defect density in comparison with the front face, the GaN substrate is bent downwardly. Such a deformation phenomenon of the GaN substrate may render it difficult to implement with respect to high-density integrated devices formed on a GaN substrate.